1. Field of the Invention
The invention relates to a semiconductor multiple quantum well Mach-Zehnder optical modulator having a low driving voltage and a low optical wave guide loss and a method for fabricating the same.
2. Description of the Related Art
An optical modulator is expected as one of the most important elements constituting optical communication system is and optical information processing systems. In various kinds of the optical modulators, a Mach-Zehnder type optical modulator receives a great deal of attention and expectation due to no wave form deterioration due to a chirping in the modulation light. A high speed and long distance optical transmission is mainly limited by the wave form deterioration due to the chirping in the modulated light. The wave form deterioration due to the chirping in the modulated light is remarkable as the optical transmission speed and the optical transmission distance are large. For those reasons, the Mach-Zehnder type optical modulator free from any chirping problem is expected as an optical modulator permitting a high speed and long distance optical transmission.
When a semiconductor is placed in an electrical field, the absorption coefficient changes with the electrical field. This phenomenon has been known as the Franz-Keldysh effect. When a bulk semiconductor is placed in an electrical field, the refraction index changes in proportion to the intensity of the electrical field. When a quantum well or multiple quantum wells are placed in an electrical field, the refraction index changes by a quantum confined Stark effect. The Mach-Zhender type optical modulator utilizes the phenomenon of the shift of the refractive index of the wave guide when applied with the electrical field.
A typical structure of the Mach-Zehnder type optical modulator comprises two passive regions each of which includes a Y-branch wave guide and an active region including two of phase modulation arms on which an electric field is applied. The Mach-Zehnder type optical modulator having a multiple quantum well wave guide structure is disclosed in 1993 electron information and communication conference, C-151, 4-187 reported by Hirohisa SANO et al. The multiple quantum well wave guide of the Mach-Zehnder optical modulator has a ridge wave guide structure. An incident light wavelength is 1.55 micrometers. The multiple quantum well structure comprises 30-periods of InGaAs/InAlAs layers wherein each InGaAs layer serves as a potential well having a thickness of 6.5 nanometers and each InAlAs layer serves as a potential barrier having a thickness of 6.0 nanometers so that a band gap wavelength is 1.45 micrometers. A total length of the Mach-Zehnder type optical modulator is 1.2. millimeters and a length of the active region of the phase modulation region on which the electrical field is applied is 0.5 millimeters. When the incidental light wavelength is 1.55 micrometers, a half wavelength voltage is 4.2 V and an extinction ratio is 13 dB and an insertion loss is 12 dB.
As described above, the multiple quantum well Mach-Zhender optical modulator utilizes a change of the refractive index of the multiple quantum well wave guide by applying an electrical field on the phase modulation arms in the phase modulation region. The Mach-Zehnder optical modulator has a smaller size as compared to a dielectric optical modulator using LiNbO.sub.3.
In view of further improvements in performances of the Mach-Zehnder optical modulator, it is very important to further reduce a driving voltage and a wave guide loss as much as possible. The Mach-Zhender optical modulator may conduct a phase modulation of light confined in the multiple quantum well wave guide constituting the phase modulation arms by application of an electrical field thereon. An amount of the phase shift of the light depends upon an intensity of the applied electrical field. The increase of the intensity of the applied electrical field requires an increase of the driving voltage. The reduction of the driving voltage of the Mach-Zehnder type optical modulator requires an improvement in efficiency of the refractive index change by application of a predetermined electrical field on the multiple quantum well wave guides in the phase modulation region of the optical modulator.
It has been known that the refractive index change in application of the predetermined electrical field on the phase modulation arms of the Mach-Zehnder optical modulator becomes large as the band gap wavelength of the multiple quantum well wave guide constituting the phase modulation arms approaches the incidental light wavelength. Notwithstanding, if the the band gap wavelength of the multiple quantum well wave guide constituting the phase modulation arms approaches the incidental light wavelength, then the wave guide loss becomes large. The large wave guide loss results in a difficulty in achieving the required long distance optical transmission. The reduction of the wave guide loss requires a large shift of the band gap wavelength of the multiple quantum well wave guide from the incidental light wavelength into a shorter wavelength range. Consequently, the phase modulation arms in the phase modulation region on which the electrical field is applied to cause an optical phase modulation is required to have such an energy band gap as corresponding nearly to or a band gap wavelength near to the incidental light wavelength.
In the conventional Mach-Zehnder optical modulator, the multiple quantum well wave guide have the uniform band gap energy or the band gap wavelength between in the active region including the phase modulation arms and in the passive regions including the Y-branches. That is why the conventional Mach-Zehnder optical modulator having the uniform band gap wavelength has a problem in obtaining both a low driving voltage and a low wave guide loss. If the band gap wavelength of the multiple quantum well wave guide is largely different from the incidental light wavelength, then the wave guide loss may be suppressed but a small refractive index change is obtained by a larger driving voltage is required to apply the necessary electrical field on the phase modulation arms for a predetermined phase shift of the light confined in the wave guide. By contrast, if the band gap wavelength of the multiple quantum well wave guide is near to the incidental light wavelength, then a large refractive index change is obtained with a low driving voltage but the wave guide loss becomes large.
Therefore, it is required to develop a novel Mach-Zehnder type optical modulator including a multiple quantum well wave guide having a difference in the band gap wavelength between the phase modulation region and the passive region including the Y-branches.